The present invention relates to a new and improved construction of radiation detector for a flame alarm which is provided with a sensor element and arranged forwardly thereof a filter arrangement permeable or transmissive for the resonance radiation of carbon dioxide.
In practice, flame alarms are exposed to various types of spurious radiation which can trigger a false alarm signal. From Swiss Patent No. 509,633 and its cognate U.S. Pat. No. 3,739,365, Swiss Patent No. 537,066 and its cognate U.S. Pat. No. 3,716,717 or Swiss Patent No. 558,577 and its cognate U.S. Pat. No. 3,940,753 there are known to the art flame alarms which, for the purpose of differentiating a flame from spurious radiation, such as daylight, solar radiation or artificial light sources, make use of the different characteristics or properties of flames. For instance, there is resorted to employing the irregular flickering of flames and the thus resultant irregular intensity fluctuations of the flame radiation, or the particular color temperature or spectral composition of the flame radiation. However, since certain spurious radiation can contain radiation constituents with similar properties or characteristics and since such spurious radiation in practice often is more intensive by several orders of magnitude than the flame radiation which is to be detected, such flame alarms are not completely foolproof against false alarms and can not be set to the highest possible sensitivity.
From French Patent No. 2,151,148 and the article of K. Nakajima, appearing in the journal "Report of Fire Research Institute of Japan", 30 (Dec. 1969), at pages 55-61, it has become known that the radiation of a flame primarily is formed of a narrow band intensity peak in the spectral range of the resonance radiation of carbon dioxide at about 4.3-4.4 .mu.m, apart from a considerably weaker wideband spectral region in the range of visible radiation and near infrared. The carbon dioxide-resonance radiation almost exclusively occurs with flames resulting from the combustion of organic materials, but hardly ever or only with decreasing intensity in the case of spurious radiation. A flame alarm which evaluates, apart from other criteria, essentially the resonance radiation of carbon dioxide therefore is considerably more foolproof against giving of a false alarm and less prone to disturbances than flame detectors which evaluate ultraviolet radiation, visible light or near infrared.
However, flame alarms evaluating the resonance radiation of carbon dioxide were associated with the disadvantage that the radiation detectors employed therein were too wideband and transmitted other parts or components of the radiation. Conventional interference filters for 4.3 .mu.m possess, for instance, sidebands which are located in near infrared or in the visible region, so that spurious radiation occurring in these spectral ranges equally is detected. Thus, Nakajima employed a filter specially manufactured by the United States firm, Optical Coating Laboratory. This special filter, while encompassing carbon dioxide-resonance radiation, still is too wideband for practical applications (3.9-5.2 .mu.m). In order to eliminate neighbouring spurious radiation, there must be used a lead selenide photoelement which is capable of eliminating radiation having a greater wavelength than 4.3 .mu.m. What is here disadvantageous is that at standard or room temperature the carbon dioxide-resonance radiation already is located at the edge of the sensitivity decrease or descent. Thus, the flame radiation cannot be fully exploited, the sensitivity of the flame alarm does not reach the optimum possible value and the radiation receiver cannot be optimumly selected. In addition, lead selenide sensors have the disadvantage that they only can be used up to about 55.degree. C. A further problem of interference filters consists in the angle dependency of the transmission curve. With an angular or oblique incidence of the radiation upon the filter the bandpass shifts to a certain extent towards the shorter wavelengths. This can lead to the drawback that with an angular incidence of the foreign or spurious radiation the flame alarm can be disturbed or malfunction.
In German Patent Publication No. 2,823,411 there has been disclosed a flame alarm whose radiation receiver contains a filter arrangement composed of a quartz filter and a germanium filter. The quartz filter defines an upper limit of the radiation at about 4.5 .mu.m, while the germanium filter blocks radiation below 1.5 .mu.m. This arrangement has the shortcoming that the absorption region of the quartz filter already extends into the resonance region of carbon dioxide, so that part of the resonance radiation is absorbed. Therefore, the sensitivity of a flame alarm of this type equally is not yet optimum.